Effects of drought and heat stresses during reproductive stage on pollen germination, yield, and leaf reflectance properties in maize (Zea mays L.)

Abstract

Drought and heat stresses are the major abiotic stress factors detrimental to maize (Zea mays L.) production. Much attention has been directed toward plant responses to heat or drought stress. However, maize reproductive stage responses to combined heat and drought remain less explored. Therefore, this study aimed to quantify the impact of optimum daytime (30°C, control) and warmer daytime temperatures (35°C, heat stress) on pollen germination, morpho-physiology, and yield potential using two maize genotypes ("Mo17" and "B73") under contrasting soil moisture content, that is, 100% and 40% irrigation during flowering. Pollen germination of both genotypes decreased under combined stresses (42%), followed by heat stress (30%) and drought stress (19%). Stomatal conductance and transpiration were comparable between control and heat stress but significantly decreased under combined stresses (83% and 72%) and drought stress (52% and 47%) compared with the control. Genotype "Mo17" reduced its green leaf area to minimize the water loss, which appears to be one of the adaptive strategies of "Mo17" under stress conditions. The leaf reflectance of both genotypes varied across treatments. Vegetation indices associated with pigments (chlorophyll index of green, chlorophyll index of red edge, and carotenoid index) and plant health (normalized difference red-edge index) were found to be highly sensitive to drought and combined stressors than heat stress. Combined drought and heat stresses caused a significant reduction in yield and yield components in both Mo17 (49%) and B73 (86%) genotypes. The harvest index of genotype "B73" was extremely low, indicating poor partitioning efficiency. At least when it comes to "B73," the cause of yield reduction appears to be the result of reduced sink number rather than the pollen and source size. To the best of our awareness, this is the first study that showed how the leaf-level spectra, yield, and quality parameters respond to the short duration of independent and combined stresses during flowering in inbred maize. Further studies are required to validate the responses of potential traits involving diverse maize genotypes under field conditions. This study suggests the need to develop maize with improved tolerance to combined stresses to sustain production under increasing temperatures and low rainfall conditions.

Keywords: drought stress; flowering; heat stress; hyperspectral reflectance; maize; pollen.

FIGURE 1

In vitro pollen germination of maize genotypes (B73 and Mo17) under individual and combined drought and heat stresses. Vertical bars denote mean ± SE. Means followed by a common letter are not significantly different by Duncan's multiple range test at the 5% level of significance. CNT, control; DS, drought stress; HS, heat stress; and DS + HS, combined drought and heat stresses (DS + HS)

FIGURE 2

Effects of drought, heat, and their combination during the flowering stage of maize genotypes (B73 and Mo17) on (a) chlorophyll (μg cm⁻²), (b) anthocyanin index, (c) stomatal conductance (mol m⁻² s⁻¹), (d) transpiration (mmol m⁻² s⁻¹), (e) PSII actual photochemical quantum yield (PhiPS2), and (f) photosynthetic electron transport rate (μmol e⁻¹ m⁻² s⁻¹). Vertical bars denote mean ± SE. CNT, control; DS, drought stress; HS, heat stress; and DS + HS, combined drought and heat stresses. Means followed by a common letter are not significantly different by Duncan's multiple range test at the 5% level of significance

FIGURE 3

Effects of drought, heat, and their combination imposed during flowering stage of maize genotype on spectral vegetation indices (a) chlorophyll index of green (CIgreen), (b) chlorophyll index of red edge (CIred‐edge), (c) chlorophyll vegetation index (CVI), (d) carotenoid index (CARI), (e) normalized difference red‐edge index (NDRE), and (f) normalized difference vegetation index (NDVI) measured at 13 days after stress. Means followed by a common letter are not significantly different by Duncan's multiple range test at the 5% level of significance. Vertical bars denote mean ± SE. CNT, control; DS, drought stress; HS, heat stress; and DS + HS, combined drought and heat stresses

FIGURE 4

Effects of drought, heat, and their combination imposed during the flowering stage of maize genotypes on (a) dead leaves, (b) green leaf area (cm²), (c) leaf dry weight (g plant⁻¹), and (d) shoot biomass (g plant⁻¹) measured at 13 days after stress. Vertical bars denote mean ± SE. CNT, control; DS, drought stress; HS, heat stress; and DS + HS, combined drought and heat stresses. Means followed by a common letter are not significantly different by Duncan's multiple range test at the 5% level of significance

FIGURE 5

Effects of drought, heat, and their combination imposed during the flowering stage of maize genotypes on (a) kernel number (plant⁻¹), (b) grain yield (g plant⁻¹), (c) single kernel weight (g), and (d) harvest index measured at physiological maturity. Vertical bars denote mean ± SE. CNT, control; DS, drought stress; HS, heat stress; and DS + HS, combined drought and heat stresses. Means followed by a common letter are not significantly different by Duncan's multiple range test at the 5% level of significance